Apparent Carrier Concentration Research Articles

Variable temperature thermal droop characteristics of 255 nm UV LED

Thermal droop, i.e., the loss of emission efficiency over a certain temperature range, is an important performance bottleneck for the successful commercial application of deep-ultraviolet light emitting diodes. In this study, we examined the mechanism of two thermal droop processes of 255 nm AlGaN quantum well light emitting diodes under temperature stresses in order to obtain steady optical output in a broad temperature range. We discovered that the increase in leakage current in the low forward bias region is accompanied by a decrease in apparent carrier concentration of quantum wells near the p side during the thermal droop process at high temperature (>300 K), indicating that the activation of thermal defects enhances the trap assisted tunneling effect and causes the optical power to decrease more significantly at low current. Compared with normal temperature, the low emission power at low temperatures is attributed to the minority trap H1, which has an activation energy of 0.527 eV at 190 K, according to deep level transient spectrum analysis. At low temperatures above 175 K, the optical power increases as the temperature rises due to enhanced hole injection. By analyzing the droop characteristics, we concluded that the activation of thermal defects is the most probable cause of high temperature thermal droop in 255 nm AlGaN quantum well light emitting diodes, whereas hole trap H1, which is linked to gallium vacancy complexes related defects, is most likely the source of low temperature thermal droop.

Impact of current-induced degradation process on the electro-optical characteristics of InGaN/GaN multiple-quantum-well photodetectors fabricated on sapphire substrate

The impact of defects on the degradation behaviors of InGaN/GaN multiple-quantum-well photodetectors submitted to dc current stress has been intensively studied. The root mechanism for degradation has been studied employing combined electro-optical measurements. The collected results indicate that (i) stress can induce an increase in parasitic current leakage paths and trap-assisted tunneling in reverse and subturn-on forward bias ranges, respectively; (ii) during stress, the overall capacitance increases and localization improves in the apparent carrier concentration within the active region; (iii) stress causes a significant decrease in quantum well intensity and an increase in yellow luminescence; (iv) stress induces an increase in the external quantum efficiency due to broadening of the space-charge region; and (v) the detectivity of the device decreases after the stress treatment. These results suggest that degradation is largely attributed to the activation of initially inactive defects, mainly Mg–H, C-related, and VGa defects in the investigated devices, with consequent worsening of their performances.

Effects of thin titanium and graphene depositions and annealing temperature on electrical, optical, and mechanical properties of IGZO/Ti/graphene/PI specimen

High-quality monolayer graphene produced on copper foil using the chemical vapor deposition method, was transferred to a polyimide (PI) substrate to improve the electrical and mechanical properties of IGZO/Ti/graphene/PI specimens. Ti film was deposited as the interlayer to protect the graphene layer during the IGZO deposition and improve the adhesion strength between the IGZO film and the graphene/PI substrate. Significant increases in hardness (H) and reduced modulus (Er) and a great reductions in sheet resistance and resistivity are obtained when the pattern integrity and microstructure of the graphene layer were remained after the IGZO deposition. The X-ray diffraction (XRD) intensity and grain size of InGaO3(ZnO)3 (0012) increase with increasing the annealing temperature and using the graphene layer. Annealing temperature rise can reduce the residual tensile stress, but also the hardness, and reduced modulus. Using graphene film as the interlayer and increasing the annealing temperature greatly reduced sheet resistance and resistivity, but significantly increased carrier mobility and concentration compared to those for a specimen without graphene. Burstein-Moss effect is exhibited in the specimens such that both the apparent optical energy gap (Eg) and carrier concentration are elevated by raising the temperature, irrespective of the use of graphene. The effects of annealing temperature and graphene on the optical properties exhibited in the seven colors were also evaluated.

Kinetically Controlled Growth of Phase‐Pure SnS Absorbers for Thin Film Solar Cells: Achieving Efficiency Near 3% with Long‐Term Stability Using an SnS/CdS Heterojunction

AbstractFacile control over the morphology of phase pure tin monosulfide (SnS) thin films, a promising future absorber for thin film solar cells, is enabled by controlling the growth kinetics in vapor transport deposition of congruently evaporated SnS. The pressure during growth is found to be a key factor in modifying the final shape of the SnS grains. The optimized cube‐like SnS shows p‐type with the apparent carrier concentration of ≈1017 cm−3 with an optical bandgap of 1.32 eV. The dense and flat surface morphology of 1 µm thick SnS combined with the minimization of pinholes directly leads to improved diode quality and increased shunt resistance of the SnS/CdS heterojunction (cell area of 0.30 cm2). An open‐circuit voltage of up to 0.3068 V is achieved, which is independently characterized at the Korea Institute of Energy Research (KIER). Detailed high‐resolution transmission electron microscopy analysis confirms the absence of detrimental secondary phases such as Sn2S3 or SnS2 in the SnS grains or at intergrain boundaries. The initial efficiency level of 98.5% is maintained even after six months of storage in air, and the final efficiency of the champion SnS/CdS cell, certified at the KIER, is 2.938% with an open‐circuit voltage of 0.2912 V.

Effects of potassium on kesterite solar cells: Similarities, differences and synergies with sodium

Addition of alkali dopants is essential for achieving high-efficiency conversion efficiency of thin film solar cells based on chalcogenide semiconductors like Cu(In,Ga)Se2 (CIGS) and Cu2ZnSn(S,Se)4 (CZTSSe also called kesterite). Whereas the treatment with potassium allows boosting the performance of CIGS solar cells as compared to the conventional sodium doping, it is debated if similar effects can be expected for kesterite solar cells. Here the influence of potassium is investigated by introducing the dopant during the solution processing of kesterite absorbers. It is confirmed that the presence of potassium leads to an enhanced grain growth and a ten-fold lower potassium concentration is sufficient for obtaining grain size similar to sodium-containing absorbers. Potassium is located predominantly at grain boundaries and it suppresses incorporation of sodium into the absorber layer. The potassium doping increases the apparent carrier concentration to ∼2×1016 cm-3 for a potassium concentration of 0.2 at%. The potassium-doped solar cells yield conversion efficiency close to 10%, on par with only sodium-doped samples. Co-doping with potassium and sodium has not revealed any beneficial synergetic effects and it is concluded that both dopants exhibit similar effects on the kesterite solar cell performance.

Degradation Mechanisms of Heterogeneous III-V/Silicon 1.55- $\mu \text{m}$ DBR Laser Diodes

This paper reports an extensive analysis of the degradation processes of heterogeneous III-V/Silicon infrared laser diodes designed for integrated telecommunications and interconnects. By submitting the devices to a series of constant current stress tests, a gradual degradation of the main device parameters was observed. In particular, in every stress scenario the devices under test showed: 1) an increase in the threshold current; 2) a decrease of the turn-ON voltage; and 3) an increase in the apparent carrier concentration within the space charge region. The variation of the electrical parameters (turn-ON voltage and apparent charge concentration) was found to be significantly correlated to the optical degradation for long stress times; the results support the hypothesis that degradation originates from an increase in the non-radiative recombination rate, possibly due to the diffusion of defects toward the active region of the devices. In order to investigate the physical origin of those defects, a capacitance deep level transient spectroscopy analysis was performed. The results indicate the presence of five different deep levels, with a main trap located around 0.43 eV above the valence band energy. This trap is compatible with an interface defect located between the In 0.53 Al x Ga 0.47-x As separate confinement heterostructure region and the InP layer.

Influence of Indium Content on the Unintentional Background Doping and Device Performance of InGaN/GaN Multiple-Quantum-Well Solar Cells

We experimentally investigate the influence of indium composition on the photovoltaic performance of InGaN/GaN multiple-quantum-well (MQW) solar cells. Three MQW structures with different In content are grown via metal-organic chemical vapor deposition, and then fabricated into solar cell devices. The solar cells with lower In content show stronger photovoltaic response, which may be ascribed to both smaller piezoelectric fields in InGaN well layers and better material quality of MQW active region. In addition, based on the capacitance–voltage measurements by which the profiles of apparent carrier concentration can be determined, we find that in the higher-In-content solar cells the background electron concentration in unintentionally doped InGaN QWs is higher. Consequently, the effective volume of active region, or the width of depletion region, may become smaller at zero bias and correspondingly the light absorption by InGaN active layers is reduced. This may be attributed to the increased nitrogen vacancy-related defects which are easier induced into In-rich InGaN alloys as a source of shallow donors.

(Invited) Organic Polymers for Thermoelectric Energy Conversion

Semiconducting polymers have been proposed as potential materials for thermoelectric devices. While there have been significant advances in designing polymers for applications such as solar cells and thin film transistors, significantly less is understood about how to control their thermoelectric properties. The performance of thermoelectric materials is determined by their electrical conductivity, thermopower and thermal conductivity. Each of these properties is related to the carrier concentration in semiconducting materials and their electronic density of states. The electronic density of states is strongly modified by processing methods used to introduce dopants making it difficult to predict their electrical properties. We will discuss our efforts to understand how the electrical conductivity and thermopower of semiconducting polymers are interrelated using model polymer systems including poly(3-hexylthiophene) and a thienothiophene-basedpolymers, PBTTT. We find that changes in processing conditions can increase the electrical conductivity by >50x at the same apparent carrier concentration, while causing smaller changes in the thermopower. The increase in performance can be understood by the nanoscale connectivity between ordered domains and quantitated using synchrotron-based X-ray scattering methods. We will also present temperature-dependent transport measurements that further help to understand changes in performance. Prospects for the ultimate performance of polymer thermoelectrics will be discussed.

Increase of photogenerated carriers in thick quantum wells in InGaN solar cells verified by laser-assisted capacitance-voltage measurement

For the InGaN/GaN multiple quantum well (MQW) solar cells, it is known that in the thicker InGaN QWs the collection of photogenerated carriers suffer from the impact of both stronger piezoelectric field and enhanced non-radiative recombination process, which may lead to an inferior performance for thick-well solar cells. However, we find that the photovoltaic intensities of our MQW solar cells with different well thickness are almost the same. Combined with the profiles of apparent carrier concentration obtained from the capacitance-voltage measurement with additional laser illumination, we conclude that the increase of photogenerated carriers in the thick QWs may partially counteract the adverse effects of both polarization effect and deteriorated material quality on the photocurrent for the MQW solar cells with thick InGaN well layers, compared with the thin-well ones.

Rational synthesis of (Cu1−xAgx)2ZnSnS4 nanocrystals with low defect and tuning band gap

Kesterite Cu2ZnSnS4 has emerged as promising replacement for CIGS in scalable commercial thin film photovoltaics. Due to the chemical and size similarity of Cu and Zn atom, [CuZn+ZnCu] defect cluster has low formation energy and high concentration in CZTS, which resulting large amount of recombination centers. In this manuscript, (Cu1−xAgx)2ZnSnS4 nanocrystals were rationally synthesized with low defect and tuning band gap. When 2% of Cu was substituted by Ag, highest apparent carrier concentration and lowest defect were observed. The band gap of (Cu1−xAgx)2ZnSnS4 nanocrystals can be rationally controlled by adjusting Ag/(Cu+Ag) ratio. This work may contribute to enhancing the performance of Cu2ZnSnS4-based solar cells and photocatalyst in hydrogen evolution.

Capacitance‐voltage and admittance investigations of InGaN/GaN MQW LEDs: frequency dependence

AbstractCapacitance‐frequency dependences for InGaN/GaN multiple‐quantum‐well light‐emitting structures grown by MOCVD are investigated at various temperatures and bias voltages. The charge relaxation in a quantum well is found to be satisfactorily described by two emission processes with different dependences of emission rate on temperature. It is shown that in typical InGaN/GaN lightemitting structures, one or a few quantum wells can be filled with electrons even for the relatively high reverse biases applied to the structure and contribute to the measured capacitance. This allows to explain the observed temperature and frequency dependences of apparent carrier concentration profiles obtained under capacitance–voltage profiling in these structures. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of the temperature dependence of the capacitance-voltage characteristics of InGaN/GaN multiple quantum well light-emitting structures

The capacitance-voltage characteristics and frequency dependences of the capacitance and conductance of InGaN/GaN multiple quantum well light-emitting structures are studied in the frequency range of 60 Hz-5 MHz and the temperature range of 77–300 K. It is shown that carrier relaxation in quantum wells can be described by two emission processes, i.e., the thermally activated one and with the power-law temperature dependence of the emission rate. It is also shown that one or several quantum wells in typical InGaN/GaN-based light-emitting structures can remain filled with electrons even at comparatively high reverse biases. This makes it possible to explain the depth shift of the apparent carrier concentration profiles, obtained from the capacitance-voltage characteristics, with decreasing temperature.

Capacitance-voltage profiling on polar III-nitride heterostructures

It has been long recognized that the capacitance-voltage (C-V) profiling technique only yields an apparent carrier concentration n̂, which, in general, differs from the majority carrier concentration. However, it is still possible to extract the conduction band offset ΔEc and interface charge σi from n̂, provided the doping profile and the exact hetero-interface position are known. We will show that in the case of σi∼1012cm−2, which is typical for the polar III-nitrides, an interface misplacement as small as 1 nm leads to a large error in the extracted ΔEc, making the technique challenging to use, even with the help of a computer-assisted reconstruction. We used one-sided p+-n junctions to experimentally investigate σi and ΔEc for Ga-face n-N GaN/AlGaN heterostructures through C-V profiling. The extracted σi matched the polarization charge reported by others.

Raman spectroscopic determination of electron concentration in n-type GaInAsSb

Phonon-plasmon coupled mode Raman spectra of n-type GaInAsSb were measured at room temperature as a function of electron concentration. A relatively simple spectral model for the electronic contribution to the dielectric function was evaluated to determine the electron concentration from the bulk coupled mode spectra. The electron concentration was determined from a Raman spectrum by minimizing the sum of the squared residuals between a measured and a simulated spectrum. The only two fitting parameters were the Fermi energy and a plasmon damping parameter. The electron concentrations determined from the fits to the Raman spectra were compared to the electron concentrations determined from single magnetic field Hall effect measurements that were corrected to account for carriers in two conduction band minima. Compared to the results obtained from the Hall effect measurements, the electron concentrations obtained using Raman spectroscopy were as much as ≈19% lower at low doping levels but not more than ≈1% higher at higher doping levels. At lower carrier concentrations, the deviations are attributed to limitations of the spectral model. At higher carrier concentrations, the two methods were in good agreement. However, given the known limitations of this relatively simple spectral model, this agreement may be fortuitous; i.e., elements of the spectral model that tend to increase the apparent carrier concentration may be offset by elements that decrease the apparent carrier concentration.

Characterization of asymmetric GaN/InGaN multiple quantum well

AbstractThe microstructure and electronic structure of the asymmetric GaN/InGaN multiple quantum well (MQW) light emitting diodes are characterized by transmission electron microscope (TEM), cathodoluminescence (CL) and capacitance‐voltage (C‐V) measurements. In TEM images, the asymmetric structure of InGaN/GaN MQW are observed as designed with different width of QWs and barriers, and indium content in QWs as well. In high‐resolution TEM images, the InGaN quantum wells and GaN barriers show the same lattice constant, and some degradation can be seen at the bottom of each InGaN QW. There are two emission peaks, 450 and 530 nm in CL spectrum, similar to the ones in electroluminescence spectrum. The double emission peaks are assigned to the irradiative recombination in different InGaN QWs. From the C‐V data, the apparent carrier concentration in the asymmetric MQW is calculated. There are five obvious peaks which are well explained by an energy band scheme of InGaN/GaN MQW. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

C-V and DLTS Analyses of Trap-Induced Graded Junctions: The Case of Al<sup>+</sup> Implanted JTE p<sup>+</sup>n 4H-SiC Diodes

Capacitance versus Voltage (C-V) and Deep Level Transient Spectroscopy (DLTS) measurements of Al+ implanted p+n diodes with Al+ implanted Junction Termination Extension are here studied. These diodes present C-V characteristics like graded junction for low forward bias values, i.e. > 0.4 V , or like abrupt junctions for large reverse bias, i.e. between 0.4V and -10V. The depth range of the graded junction, computed by the capacitance values, is much larger than the simulated tail of the ion implanted Al+ profile. DLTS spectra have been measured both in injection and standard configuration and always show minority carrier traps in the temperature range 0-300K. Three are the minority carrier related peaks, one attributed to the Al acceptor and the others to the D and D1 defects. The depth distribution of these hole traps will be discussed with respect to the apparent carrier concentration, obtained by C-V analysis.

Electrical characterization of InAs thin films

AbstractIt is known that parallel conduction as a result of surface and /or interface charge accumulation significantly shields the bulk electrical properties of InAs thin films when characterized using Hall measurements. This parallel conduction in InAs can be modeled by using the two‐layer model of Nedoluha and Koch [Zeitschrift für Physik 132, 608 (1952)]; where an InAs epilayer is treated as consisting of two conductors connected in parallel viz. a bulk and a surface layer. Here, this two‐layer model is used to simulate Hall coefficient and conductivity data of InAs thin films ranging from strongly n‐doped (n = 1018 cm–3) to strongly p‐doped (p ∼ 1019 cm–3) material. Conventional Hall approximations, i.e. those that assume uniform conduction from a single band, are then used to predict the apparent carrier concentration and mobility that will be determined from conventional Hall measurements, with the aim of illustrating the error of such a simplified analysis of InAs Hall data. Results show that, in addition to ignoring parallel conduction, the approximations of conventional Hall data analysis have a further inadequacy for p‐type InAs, in that the high electron to hole mobility ratio in InAs is not taken into account. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Determination of Band Discontinuities in Silicon Heterostructures

An estimation for the band offsets and the fundamental band gap has been presented for several silicon-germanium based heterostructures. This estimation considers both the band lineup at the interface of two different materials as well as strain effects. The valence and conduction band offsets in strained Si/SiGe, partially strain-compensated Si/SiGeC and Si/strained-Si heterostructures have been determined from quantum confinement of carriers measurements. High-frequency capacitance-voltage (C-V) profiling has been used to measure apparent carrier concentration and band offsets. Extracted experimental values are found to be in a good agreement with reported values obtained using other techniques.

Electronic properties of a 3C∕4H SiC polytype heterojunction formed on the Si face

The authors report on the electronic properties of a rectifying Si face 3C∕4H SiC heteropolytype junction on n+ 4H SiC. Capacitance-voltage profiling of the junction at temperatures from 4–300K showed high apparent carrier concentration. A semiclassical model was used to explain the behavior. The model predicted a spontaneous polarization-induced valence band quantum well in the 3C, indicating a polarization charge of 9.7×1012cm−2 for 4H SiC, in good agreement with theory. The formation of a two-dimensional hole gas was predicted. Using a Poisson-Schrödinger solver to analyze the measurements, it was found that large (∼3.5×1012cm−2) mobile hole charge was induced in the n-doped 3C SiC.

Electrical characterization of InAs/GaAs quantum dot structures

The electrical properties of InAs quantum dots (QD) in InAs/GaAs structures have been investigated by space charge spectroscopy techniques, current–voltage and capacitance–voltage measurements. Au/GaAs/InAs(QD)/GaAs Schottky barriers as well as ohmic/GaAs/InAs(QD)/GaAs/ohmic structures have been prepared in order to analyze the apparent free carrier concentration profiles across the QD plane, the electronic levels around the QD and the electrical properties of the GaAs/InAs(QD)/GaAs heterojunction. Accumulation and/or depletion of free carriers at the QD plane have been observed by Capacitance–Voltage ( C– V) measurements depending on the structure parameters and growth procedures. Similarly, quantum dot levels which exhibit distributions in energy have been detected by Deep Level Transient Spectroscopy (DLTS) and Admittance Spectroscopy (AS) measurements only on particular structures. Finally, the rectification properties of the InAs/GaAs heterojunction have been investigated and the influence of the related capacitance on the measured capacitance has been evidenced.